Re: [PATCH v3] drm/dp_mst: Fix W=1 warnings

From: Dave Airlie <airlied@gmail.com>
To: Lyude Paul <lyude@redhat.com>
Cc: Benjamin Gaignard <benjamin.gaignard@st.com>,
	David Airlie <airlied@linux.ie>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	ML dri-devel <dri-devel@lists.freedesktop.org>,
	Sean Paul <sean@poorly.run>
Subject: Re: [PATCH v3] drm/dp_mst: Fix W=1 warnings
Date: Fri, 31 Jan 2020 09:22:00 +1000	[thread overview]
Message-ID: <CAPM=9txafoQNfMFf0Ff1SnBTgq6jYyvjJyjCSJua6-SJVVkScQ@mail.gmail.com> (raw)
In-Reply-To: <08f4b69b1e48a81e90f28e7672da15cc5165969c.camel@redhat.com>

> > > >
> > > hi-actually yes, we should probably be using this instead of just dropping
> > > this. Also, I didn't write this code originally I just refactored a bunch
> > > of
> > > it - Dave Airlied is the original author, but the original version of this
> > > code was written ages ago. tbh, I think it's a safe bet to say that they
> > > probably did mean to use this but forgot to and no one noticed until now.
> >
> > Hi,
> >
> > Any clue about how to use crc value ? Does it have to be checked
> > against something else ?
> > If crc are not matching what should we do of the data copied just before ?
>
> We should be able to just take the CRC value from the sideband message and
> then generate our own CRC value using the sideband message contents, and check
> if the two are equal. If they aren't, something went wrong and we didn't
> receive the message properly.
>
> Now as to what we should do when we have CRC mismatches? That's a bit more
> difficult. If you have access to the DP MST spec, I suppose a place to start
> figuring that out would be checking if there's a way for us to request that a
> branch device resend whatever message it sent previously. If there isn't, I
> guess we should just print an error in dmesg (possibly with a hexdump of the
> failed message as well) and not forward the message to the driver. Not sure of
> any better way of handling it then that

Yeah I think this reflects what I wanted to do, I've no memory of a
retransmit option in the spec, but I've away from it for a while. But
we'd want to compare the CRC with what we got to make sure the are the
same.

Dave.
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